The present invention relates to a power supply device and a discharge lamp lighting device.
An example of conventional discharge lamp lighting devices of this type is disclosed in Japanese Patent Laid-open No. 211774-1993, wherein a full-wave rectifying circuit is connected to a commercial AC power supply. A parallel resonance circuit, which comprises a resonance capacitor and an inverter transformer, and a transistor, are connected in series with an output terminal of the full-wave rectifying circuit, thereby forming an inverter circuit. In addition, a partially smoothing circuit, which includes a series circuit comprising a charge capacitor, an inductor and a first diode, and also includes a second diode is disposed between and connected to the first diode and the transistors connected to the full-wave rectifying circuit. A fluorescent lamp is connected to the inverter transformer
With the configuration as the fluorescent lamp is turned on with high-frequency AC current by means of high frequency switching of the transistor, thereby inducing high frequency AC voltage in the inverter transformer.
When the transistor is turned on while the voltage on the full-wave rectifying circuit is high, the charge capacitor is charged through the circuit formed by the full-wave rectifying circuit, the charge capacitor, the second diode, the transistor and back to the full-wave rectifying circuit.
When the transistor is turned off, the magnetic energy stored in the inductor continues to flow through the second diode. When the magnetic energy of the inductor is discharged, the second diode is turned off so that the voltage on the transistor is applied to the second diode. At that time, according to the duty ratio of the transistor, a voltage lower than the output voltage of the full-wave rectifying circuit is charged into the charge capacitor. When the input voltage becomes lower than the charging voltage on the charge capacitor, the charge capacitor discharges its stored energy through the first diode and the inductor to the inverter circuit. A substantially constant DC voltage is supplied to the inverter circuit, with its power factor improved to approximately 0.9.
Due to an insufficient voltage from the charge capacitor and the partially smoothed circuit, however, the discharge lamp lighting device disclosed in Japanese Patent Laid-open No. 1211774-1993 presents some problems in its crest factor (the ratio of peak value to effective value of a periodic wave).
Furthermore, when the transistor was switched off with the capacitor charged, the magnetic energy of the inductor permits the current which flowed through the inductor and charged the charge capacitor to flow through the inductor, the second diode, the resonance capacitor and the charge capacitor. Therefore, energy other than the oscillation energy of the inverter circuit flows into the resonance capacitor
When the second diode is in the `on` state, the voltage on the first diode is equal to the voltage on the transistor, and the voltage minus the charge voltage on the charge capacitor is applied to the inductor. Therefore, since it is necessary to increase the voltage resistance capacities of these elements, the conventional device described above presents another problem in that reducing the size of the device is difficult.